1. Field of the Invention
The present invention relates to an engine cooling system, in particular, relates to a cooling system for a liquid-cooled engine equipped with an additional thermostat downstream of a cylinder block water jacket in addition to a radiator thermostat, which system allows a parallel coolant circulation through a cylinder block water jacket and through a cylinder head water jacket when the engine is hot and allows a coolant circulation through only the cylinder head water jacket when the engine is cold.
2. Description of the Prior Art
Recently, there have been proposed and developed various engine cooling systems which can improve an engine warm-up performance and/or an engine cooling action. For instance, Japanese Patent Provisional Publication (Tokkai Showa) No. 58-162716 teaches the provision of an additional thermostat which is disposed downstream of a cylinder block water jacket to block a flow of cooling water (coolant) flowing therethrough, when a sensed cooling water temperature is below a predetermined temperature, such as during cold starts. The conventional cooling device for a water-cooled engine described in the Japanese Patent Provisional Publication No. 58-162716, would suppress excessive cooling of cylinder blocks while properly cooling the cylinder head, by opening and closing the additional thermostat depending on the coolant temperature sensed at the outlet passage of the cylinder block water jacket. Thus, the above-noted prior art device allows a cylinder head, cylinder walls, pistons, valve seats, and other working parts to more quickly reach to operating temperatures, thereby shortening an inefficient cold-operating time, and minimizing frictional loss between the pistons and the engine cylinders, and maintaining the working parts at efficient, but not excessive, temperatures. However, in the above-noted conventional device, since the thermosensitive portion of the thermostat is provided at the outlet passage of the cylinder block water jacket, with the engine cold and the thermostat thus closed, the flow of water flowing around the thermosensitive portion is disturbed, thereby causing stagnant cooling water at the outlet passage of the cylinder block water jacket. Under this condition, an accuracy of sensing a temperature of the coolant coming from the cylinder block water jacket is lowered. That is, with the thermostatically controlled valve closed, the coolant temperature cannot be sensed precisely by the thermosensitive portion of the thermostat owing to the stagnant coolant standing at the outlet passage of the cylinder block water jacket. Therefore, in the event that the coolant temperature sensed by the thermosensitive portion is less than the actual temperature of coolant flowing through the cylinder block water jacket and thus the valve-opening operation of the thermostat is retarded, there is a possibility of over-heat of the cylinder liners. In contrast to the above, when the coolant temperature sensed by the thermosensitive portion is greater than the actual coolant temperature coming from the cylinder block water jacket and thus the valve-opening operation of the thermostat is advanced, the cylinder walls will be cooled undesirably. In this case, there is a possibility that engine thermal efficiency is lowered and engine warm-up performance is lowered when the engine is warming up or cold.
Japanese Utility Model First Publication (Jikkai Showa) No. 63-96227 discloses a cooling water return passage arrangement for a V-type engine in which engine cylinders are arranged in two rows, or banks. In the V-type engines, engine cooling is traditionally achieved by circulation of coolant flowing through two banks of cylinder block water jackets and cylinder head water jackets. As is generally known, the coolant used for cooling the right and left banks flows through the radiator to the water pump. In the coolant return passage arrangement of the Japanese Utility Model First Publication (Jikkai Showa) No. 63-96227, the coolant return passage is arranged so that a return passage of coolant coming from either one of right and left banks joins with a return passage of coolant coming from the other bank at a confluence provided at the outlet of the water jacket of the other bank. Thus, coolant coming from the water jackets would be returned efficiently to the radiator. In this prior art, since two banks are offset to each other and the confluence or junction is arranged at an end of one backward projected bank of cylinder block and additionally owing to a greater effective cross-sectional area of the coolant passage at the confluence, the entire length of the V-type engine tends to be considerably increased.
On the other hand, Japanese Utility Model First Publication (Jikkai Heisei) No. 1-65993 teaches the provision of a plurality of thermostats in the coolant passage between the cylinder head and the inlet hose of the radiator, in order to more precisely control the flow rate of coolant flowing through the radiator. However, in the prior art thermostat arrangement described in the Japanese Utility Model First Publication No. 1-65993, thermosensitive portions of the respective thermostats are arranged in parallel with a stream line of the coolant flowing through the return passage to the radiator so that the thermosensitive portions are aligned with each other on the center line of the return passage. In general, the upstream thermostat is more sensitive to the coolant temperature as compared with the downstream thermostat, owing to the in-line arrangement of the plural thermostats along the center line of the coolant passage. Thus, there is a greatly increased tendency for the valve-opening operation of the upstream thermostat to become advanced. In other words, there is a greatly increased tendency for the valve-opening operation of the downstream thermostat to become retarded.